Citation:

Mayer, P., S. Duan, S. Kaushal, B. Wessel, AND T. Johnson. Environmental Trade-Offs of Stream Restoration for Managing Stormwater and Nutrients in Urban Ecosystems. AGU, Washington, DC, December 10 - 14, 2018.

Impact/Purpose:

Nutrient enrichment of water bodies continues to be a significant risk globally to human health and ecosystems. Regenerative Stormwater Conveyance (RSC) is a stream restoration approach considered a best management practice to control nutrients in urban ecosystems. However, little information is available to document the efficacy of this technique or whether nutrient credits under the Chesapeake Bay TMDL should be granted to entities utilizing this technique. This study examines RSCs effects on nutrients (P and N) and trace metals at two RSC sites in the Washington DC area, using combined field and lab measurements. Preliminary field measurements showed no consistent longitudinal change in any water quality parameter across sites. However, lab experiments showed consistent removal of N and P when sediment was amended with wood chips and leaf litter. Nutrient management effectiveness in RSCs may depend upon quantity and quality of organic matter added to the stream bed and from the adjacent riparian zone.

Description:

Hydrologic modifications in urban ecosystems impact water quality via soil compaction, impervious surface cover, drainage, and channelization. Urban stream restoration strategies that are designed to regain lost ecosystem function such as nutrient uptake may have unintended consequences and environmental trade-offs. Regenerative Stormwater Conveyance (RSC) is a restoration approach based on engineering stream channels to incorporate shallow pools, riffles, and grade controls to increase storm water retention and control erosion, and adding a carbon layer (e.g. wood chips) to the stream bed to enhance microbial processes like denitrification. We examined RSC effects on nutrients (P and N) and trace metals (Fe and Mn) at two sites 5 and 7 years post-construction, using combined field and laboratory measurements. Field measurements showed that RSCs usually had lower dissolved oxygen (DO) and pH relative to nearby untreated stream reaches, but did not have consistently different P, N, Fe, Mn, or dissolved organic carbon (DOC) suggesting that engineering ecosystem function like nutrient uptake is affected and ultimately limited by watershed variability. In lab simulation experiments, we observed removal of N and P when sediment was amended with wood chips and leaf litter, and that these DOC sources had significantly different effects on nutrient and metals release suggesting that organic matter additions to streams drive nutrient transformation and that trade-offs exist between P and N management in restored streams depending on quantity and quality of DOC source and anoxic conditions in the hyporheic zone. An additional trade-off occurs in some RSCs where elevated groundwater levels expand the zone of anoxia but, in turn, drown and kill riparian zone trees, potentially reducing DOC inputs. Consideration of trade-offs and unintended consequences are critical for choosing and implementing BMPs in urban ecosystems that will be most effective at removing nutrients and metals.

Record Details: